Federal MPs are debating the merits of same-sex marriage legislation, but it appears unlikely to go to a vote until later in the year.

Labor MPs have been given a conscience vote on the legislation, but Coalition MPs are expected to follow the party's position on the issue, which does not allow for any change to marriage laws.

There are currently two separate bills before the House of Representatives aimed at legalising same-sex marriage - one from Greens deputy leader Adam Bandt, and the other from Labor backbencher Stephen Jones.

Supporters of gay marriage are urging Opposition Leader Tony Abbott to give his MPs a free vote on the issue so they can vote according to their conscience.

"I think it would be wise to avoid having the vote until such time as Tony Abbott decides to give his own members a conscience vote," Greens leader Christine Milne said.

"The vote shouldn't take place until at least after the winter break."

The divide

Debate on the issue has again exposed divisions within political parties - most notably on the Labor side.

Labor backbencher Graham Perrett, who has two gay brothers and three heterosexual brothers, chaired a parliamentary inquiry into the issue, and strongly backs changes to the Marriage Act.

"It is important to remember that God did not write the Marriage Act," Mr Perrett wrote in his report.

"It is written by lawyers and legislators and must reflect the views and values of today.

"The love between same-sex couples is no different to that of opposite-sex couples, and deserves no less the public recognition, celebration and symbolism which the wonderful institution of marriage bestows."

But fellow Labor backbencher John Murphy disagrees, saying the push for same-sex marriage is not about equality or human rights, and should be reserved for "people of the opposite sex".

"I have found that most of the support for same-sex marriage has come from people who are heterosexual - particularly young people, including teenagers who have abstract and less developed notions of equality, based on inexperience," Mr Murphy said.

And he says some supporters of gay marriage were "downright abusive and intolerant" of his view when raising the matter with him.

Labor MP Shane Neumann says framing the debate around marriage equality has been a "clever marketing ploy" by backers of same-sex marriage, but he argues that is "disingenuous" because there are very good reasons for restricting or prohibiting certain types of marriage relationships - including those involving children and close relatives.

"I believe it is a fundamental institution of our society - a marriage between a man and a woman," Mr Neumann said.

"Before we mess with this concept of marriage - which is held by so many Australians - we need widespread and strong community consensus. We don't have it."

'Attempts to limit love will fail'

Seven people have so far spoken as part of the debate on legislation to legalise same-sex marriage - but only one in support of the changes.

Mr Bandt, who introduced one of the bills, says MPs have an "obligation" to send a message to same-sex couples that their relationships are valued.

"The Australian population has moved on," Mr Bandt said. "The Australian population is relaxed about someone marrying the person that they love.

"There have been many attempts throughout history to limit love and all have failed.

"And as we move further into the 21st century, I am confident that attempts to limit love will fail yet again and that full marriage equality will become a reality."

But without a conscience vote on the Coalition side, Mr Bandt's legislation will not be able to pass the House of Representatives.

"If there was a conscience vote on this issue - subject to the legislation being satisfactory and going through it - I would be more likely to support it than not," Liberal frontbencher Malcolm Turnbull has told Sky News.

"But the fact is, that politics is a team sport. We have decided on our side not to have a conscience vote... so I won't be crossing the floor on this issue."

Debate on a separate piece of legislation aimed at introducing same-sex marriage laws will resume tonight.

In her resignation from politics, Kelly O'Dwyer said she feared another miscarriage in Canberra, far from home. Her announcement is shocking for more than just party-political reasons, writes Emma A. Jane.